Heretofore, secondary battery systems for detecting the state of secondary batteries have been proposed in various types. For example, according to Patent Literatures 1-4, the state of charge (the amount of stored charge (storage amount) or SOC) of a secondary battery is detected based on the battery voltage of the secondary battery. Patent Literature 4 further shows that the state of degradation of a secondary battery is also detected based on a SOC (State Of Charge) calculated based on the battery voltage, and based on a battery temperature detected by temperature detection means.